Impound is when the cars are impounded after qualifying. No practice or major adjustments allowed. This is new this year, and is only done at certain races. Why they only do it at certain races is beyond me.
Last year you would have 1 practice, qualifying then two practices to make adjustments etc. You could work on your just about up to the race.
Impound race, you get two practices then qualifying then your car is impounded and you cannot touch it till right before the race. And then you can only make minor adjustments like Air pressure, tape on the nose, wedge and track bar. You can't change springs, shocks, camber etc. Like you can in a non impound.
It makes it tougher on the teams to the balance of a car that will qualify well and race well. Puts more of an emphasis on adjustments during the race instead of in the garage before the race.
In my opinion it favors the veterans a little more.
You can tell if its impound or not just by looking at the weekend schedule.